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Section 4 <br />Potential for Rock to be Toxic-Forming <br />groundwater in the area is included in Hydrogeologic Evaluation of'the Sunday Mines,Groupj.,San , <br />Miguel, Colorado (CDM 2009c). 1 1". <br />4.2.2 Volume of Percolation through Rock Piles <br />The screening-level analysis presented previously shows that under the water to rock ratios <br />used in the MPMP analyses, leachate from the piles would be expected to contain <br />concentrations of several metals such that the development rock and ore might be considered <br />toxic-forming. To further evaluate if the development rock should be considered as a toxic- <br />forming material, this section evaluates the potential for the rock piles to generate leachate in <br />significant volumes. This is a factor of the climate of the area and the physical characteristics of <br />the piles. Two methods were used in this analysis: <br />¦ A water balance approach; and <br />¦ Unsaturated flow modeling. <br />Water Balance Estimation of Percolation: This calculation estimates percolation through the <br />Sunday Mine Group development rock areas using a water balance approach (adapted from <br />Guymon 1994). To clarify water balance terms, percolation is the volume of water that <br />discharges from the base of the rock pile measured as cubic inches percolation per square inches <br />of rock pile footprint (i.e., inches). Infiltration is the volume of water that infiltrates the surface <br />of the rock pile measured as cubic inches percolation per square inches of rock pile footprint <br />(i.e., inches). Water that infiltrates into the pile may be subsequently removed from the pile by <br />evaporation or transpiration or the water may move downward in the pile and become • <br />percolation. <br />Empirical information such as temperature and precipitation data and development rock <br />characteristics are used for the water balance evaluation. The procedure included the following <br />activities: <br />¦ Compile temperature and precipitation data from a location as close as possible to the site; <br />¦ Estimate runoff and evapotranspiration (ET) for the rock pile; <br />¦ Estimate moisture holding capacity of development rock based. on site specific capillary <br />moisture data obtained from soil water characteristic curve (SVVCC) testing; and <br />¦ Estimate percolation through the development rock pile using a water balance approach. <br />Details of the estimation method are included in Attachment 5 of this report. As discussed <br />previously, development rock samples were collected for physical property analyses including <br />SWCC testing. Geotechnical laboratory data for the development rock samples, including <br />SWCC testing data, are also provided in Attachment 5. Exhibit 4-1 presents the results of <br />monthly percolation estimates for an average precipitation year. <br />• <br />cm 4-10 <br />TA64986-Denison Mines\Task Order 3 - DMO Sampling and Analysis Plan%Task 3.12 - Soil, Ore, Rack Data Assessment ReportlSunday Rock Reporflfinal reportlReportlFinal Sunday Mines Group Sail Ore Rock <br />Date Assessment Report_051509.doc